A thermometer is a device that tells you how hot or cold something is. There are many different types of thermometers for different heat ranges. Some tell you how hot you are so you know if you are so sick you are running a fever. Some tell you what temperature the air is, and others tell you how hot your oven is. The thermometer in the picture works by the expansion and contraction of mercury as the temperature changes, but most thermometers today are digital and simply display numbers.